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VLANs
A switch that supports VLAN provides the capability to partition the
switch's ports into artificial Layer 2 broadcast domains. A VLAN
behaves exactly like a physical Layer 2 broadcast domain. Unicast,
broadcast, and multicast traffic sent into the VLAN are only visible to
ports and the hosts connected to those ports that are designated for
that VLAN. Also, as with traditional Layer 2 broadcast domains, it is
possible to bridge between VLANs.
Initially, VLANs were introduced on a proprietary basis by several
different switch vendors. The technology was seen as a way of
maximizing the usefulness of the switch's port density by enabling it
to support multiple networks on the same switch in a secure manner.
That capability was welcomed with open arms by many network
administrators because early switch products were expensive and
VLANs made it possible to provide switching to different Layer 3
segments with a single Layer 2 device. In terms of host management,
VLANs promised tremendous flexibility when a workstation needed to
be moved from one network to another network (utilizing different
Layer 3 addressing), instead of "repatching" in the wiring closet.
Using VLANs, the workstation's port could just be reassigned to the
VLAN corresponding to the new network. Along these same lines,
VLANs made it possible for workstations in different geographical
locations within the enterprise to be on the same local network
segment. VLANs also provided mechanisms to control broadcast and
multicast traffic, which can be a limiting factor in growing switched
LANs. These mechanisms made it possible to utilize high-speed switch
ports as a packet-switched backbone link that shared the port's
available bandwidth between different VLANs.
Of course, like many new technologies, VLANs had a rough start.
Various processing chores needed to maintain the VLANs in the initial
implementations sacrificed most, if not all, of the performance gained
from using switches. This problem, however, was overcome with the
development of faster ASICs. Another rather cumbersome problem
was managing VLAN construction and membership; although this
problem has been somewhat addressed, it is not yet fixed.
Port-Based VLANs
The simplest and most common method of addressing the problem is
with port-based VLANs. With port-based VLANs, the switch's ports are